Join NLA actions from multiple objects
I dont think there is an option for that.
Action is per object. So to have them in a single action you will need to parent them to Armature (each object to a single bone) and animate.
Select Object &gt; Select Armature &gt; Switch to Pose mode Ctrl+Tab &gt; Parent to Bone Ctrl+P

In a current situation the only one thing that can help you a bit is increase Repeat option in NLA Properties &gt; Strip tab &gt; Action Clip panel for selected strip.

Note - In a case you will have more tracks for one object you can search for Bake Action to merge them by step 10 here.